The
conspicuous domed eggs are laid singly on the upperside of sallow
leaves. The usual foodplant is broad-leaved sallow
Salix caprea but the narrow-leaved
S. cinerea is also
sometimes used.

In July 2005 I watched
a female which in the course of 3 or 4 minutes laid about 10 eggs at
various heights between 2 and 5 metres on broad-leaved and
narrow-leaved sallows growing on both sides of a track in a Wiltshire
wood. The eggs were laid on semi-shaded leaves in the interior of the
trees. At other sites in Hampshire and Surrey I have watched females
ovipositing on low, sunlit leaves on woodland edge sallows; and at a
thicket in north-east Hampshire I have seen a female oviposit at
eye-level on a totally shaded broad-leaved sallow.

When first laid the eggs
are pale green, but after about 5 days they develop a dark purplish
band near the base. They hatch after about 14 days and the newly
hatched larva makes it's first meal of the eggshell.

The
young larva is greyish-brown and has 2 prominent horns on its head. It
feeds until October and then enters hibernation while in the 2nd or
3rd instar. It spends the winter resting on a silk pad spun on the
upperside of a withered sallow leaf, or in the fork of a twig on the
damper and shadier north or east facing side of the tree. It resumes
feeding in early April.

When fully grown in mid-June the larva is
plump and green. It is marked along the sides with dark-edged diagonal
cream stripes that perfectly simulate the veins of a leaf. Two horns
project forward from the head, and the body is strongly tapered at the
tail. It is quite unlike that of any other British butterfly or moth.

The larva rests by daytime along
the midrib on the upper surface of a leaf. It periodically nibbles
at the tissue either side, leaving the midrib and leaf tip intact.
At dusk it vacates it's resting place and wanders all over the tree
to feed. It lays a near-invisible trail of fine silk along its
route, and uses this as a map to return to its 'home' leaf before
dawn.The chrysalis,
which wriggles frantically like a wet fish if touched, is a
beautiful shade of translucent silvery green, marked on the abdomen
with short whitish diagonal dashes. It is slightly flattened in
shape. The camouflage is so perfect that it is virtually impossible
to locate, as it hangs suspended by the cremaster from a sallow
leaf. The pupal stage lasts for about 14 days.Adult behaviour

The
butterflies begin emerging in late June and early July, although they
tend to emerge a week or two later if the early summer has been cool.
They remain on the wing until early August.

Purple Emperors spend
most of their time resting high in trees, the males favouring oaks
while the females more often rest in tall sallows. Females in
particular may spend an hour or more on their tree-top perches,
especially if the weather is cloudy or breezy. When the sun appears,
even if only for a brief period, the males take flight and circle
around the vicinity in search of food sources.

Both sexes will feed
at aphid secretions which often coat the upper surface of sallow or
oak leaves. They are also strongly attracted to sap runs. Males
additionally feed at carrion, but are most often encountered when
imbibing fluids from carnivore dung or urine-soaked ground. These
provide them with essential alkaloids which are passed to females
during copulation. Females are sometimes
seen imbibing moisture from forest tracks between bouts of egg-laying,
but do not visit carrion or dung.

In July 1984 I found
no less than 6 males feeding at the carcass of a roe deer that was
floating in an open cesspit in a Hampshire thicket. The butterflies
were so stupefied by their unsavoury meal that 2 of them remained on
the carcass as I lassoed a rope around the antlers and hauled it to
the edge of the cesspit to take photographs !

Mud-puddling and
dung-feeding activity typically begins at about 8.30am on warm sunny
mornings, or later if it is cooler or slightly overcast, and can
continue until 12.30pm or later although there is usually a lull in
activity between noon and 2pm. Sometimes a male will spend an hour or
more on the ground without moving. Further puddling sometimes occurs
in the late afternoon at about 6.00-7.00pm if it remains sunny.

In the early afternoon
both sexes fly to the highest point in the forest - the 'master tree',
typically a tall oak. As many as 6 or 7 may gather there on any
particular day. Males usually arrive first, and soon begin to chase
each other around in circles, competing to obtain the best 'throne'
- usually a prominent clump of leaves on which they sit to await the
arrival of the females.

When a
female appears the dominant male charges after her, followed by any
other males at the tree, and the butterflies fly in a string, one
behind the other, following the female as she flits and glides around
the upper branches. The strongest and most aggressive male eventually
chases off his lesser rivals, and then follows the female until she
settles on a sunlit clump of leaves, often half a kilometre or more
distant from the master tree. Copulation takes place there in early
afternoon. The mating pair spend most of the remaining afternoon
sitting on their love nest, but sometimes fly in tandem from one
branch to another, or more rarely descend to ground level. In July
1986 for example, while watching a mud-puddling male in a Surrey wood,
I saw a copulated pair float down from an oak, and settle on gravel.
When I approached they flew up into another tall oak, settling near
the top of the tree. On another occasion I spotted a copulating pair
late in the afternoon, at a height of about 6 metres in a spruce. The
pair were still copulating at 7.00pm, when I had to leave, and I
suspect that they remained joined until dusk, or possibly until the
following morning.

In some years
prolonged periods of dull or rainy weather occur during the flight
season. Then when the sun does briefly appear, a frenzy of courtship,
mating and egg-laying takes place. They are able to make the most of
very limited opportunities, e.g. weather conditions during throughout
the 2007 flight season in the UK were cool, overcast and often wet,
with precious little time for mate location or egg laying.
Nevertheless 2008 and 2009 produced the highest Purple Emperor counts
on record. Similarly in 2012 Britain experienced atrocious weather,
with cool wet and windy conditions lasting from early May until late
July. The weather then suddenly changed and became very hot and sunny
for about a week, during which time the butterflies emerged en masse,
and were able to lay a large number of eggs.

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